Here I dug up from the HONDA forum.

Expertise

OIL COMPATIBILITY

IS THIS MIXTURE?

ALEXANDER BUDKIN

Remember, dear reader, when you first heard that "mineral water" cannot be mixed with "synthetics". Let's try to guess: around the time we learned that motor oils are mineral or synthetic and that they are not the same thing at all. Have you guessed? Even more confident that you have hardly read about oil compatibility testing. Even the specialists with whom we often communicate try to bypass this topic - there are almost no universally recognized compatibility criteria, very few test methods, etc. And besides, which company is profitable to say that its oil is compatible with others. Wouldn't it be better to recommend that customers buy only their own product? In general, we decided to cheat.
WHAT TESTED
There are hardly any fans to mix two or three different engine oils in equal proportions and pour this mixture into the engine. Mixing most often occurs when replacing one oil with another, when refilling or ... flushing. Yes, yes, don't be surprised. It is often washed precisely so that the oils do not mix. When switching, say, from mineral to synthetic, a master in a car service will certainly say: necessarily with flushing. In this case, the old and new engine oils do not really mix, but mixing still occurs: first, the old oil with flushing oil, and then flushing oil with fresh one.
To check compatibility, we bought two mineral oils, two semi-synthetic, two synthetic and two flush oils. We checked each one with each other!
They were guided, like ordinary customers, by the inscriptions on the canister. Written "semisynthetic" or "synthetic blend", consider it a semi-synthetic oil without going into details. It says "fully synthetic", please represent the synthetic diaspora. So, our standard bearers have become: from mineral oils - "Spectrol Super Universal" and "Sintec" (both SF / CC levels with a viscosity of 15W40), from semi-synthetic oils - "Esso Ultra" and "Valvolin Dura Blend" (both SJ / CF, 10W40 ), from synthetic - "Castrol GTX Magnatek" and "Shell Helix Ultra" (SL / CF, 5W40), finally, from flushing - "Luxoil flushing oil" and "LUKOIL Auto Flushing" (see photo).
HOW TESTED
Since sane people do not dilute different oils, it did not make much sense to mix them in our experiments in a 1: 1 ratio. When changing the oil, the so-called non-draining residue averages about 10% (it is believed that it can vary from 5 to 20%). Therefore, the method (of the Ministry of Agriculture), which was recommended to us by experts, involves checking the compatibility of two samples after mixing them in proportions of 1: 9 and 9: 1. These mixtures are kept for a long time, or they are heated and kept, or ... However, how they are tortured, consumers do not need to know at all. It is more important to ask the same specialists how they assess compatibility.
Appearance - one, viscosity check - two, alkaline reserve - three, ash content - four ... Enough, perhaps. And so many accumulates: eight samples "each with each" multiply by two concentrations (1: 9 and 9: 1), multiply by three checked parameters - this is already several hundred digits! How do they determine compatibility? Of course, it is visually assessed whether a precipitate has formed. Then compare the three named parameters for the mixture before and after aging (appearance does not count). If they begin to deteriorate over time, this means incompatibility for an inoperative oil mixture.
It is clear that there is no reason to introduce the "two by three meters" table. Let's just talk about the most interesting results.
WHAT GOT
There are at least three such results. First and foremost, the experts stated that the technique used (and it has not existed for the first day) did not reveal any incompatibility. Optimists will exclaim “hurray, you can mix everything!”, While pessimists will probably say something very unflattering about the method. Let's try to stay "above the fight."
We have more than once turned to pundits with a prosaic question before: is it possible to ditch an engine with a mixture of oils? Each time they received a cautious answer: we are not aware of such cases. As a rule, all the rumors that someone mixed "synthetics" with "mineral water" and because of this ruined the engine are nothing more than horror stories. If Uncle Vasya screwed up the engine, it was not at all because he "interfered", but because he strove to buy cheaper oil and ran into a fake.
The test results presented here have confirmed that there are no catastrophic changes in the engine oil blend. However, as always, cautious specialists emphasize that this is a specific technique.
In other words, a mixture of two synthetic oils will not be worse for the engine than a cheap "mineral water", although it may not work like each (or at least the best) of the two samples before mixing them. But checking is a completely different task. Instead of a few hundred simple tests, a few very complex ones are required. We will probably deal with this over time, but for now we will tell you about two more results.
Of all the abundance of mixtures, three were found that, from the point of view of the layman, behaved antiscientific - the properties of the mixture did not deteriorate over time, but, on the contrary, the alkalinity increased slightly. Scientifically, this is called "synergy". It's like waves in the sea: two small ones, overlapping each other, form one big one.
So, in the pair "Esso" - "Castrol" (90% + 10%), the initial alkalinity of the samples was 6.40 and 8.56 *, after mixing it became 6.48, and with time increased to 9.90. For Valvolin (from the original 7.99) and Castrol it has changed from 8.50 to 9.11. And in another combination - 10% "Castrol" and 80% "Shell" (from the original 8.78) - it turned out respectively 8.57 and 9.80.
This suggests that some chemical processes are taking place in the mixture, but no one will undertake to say how they will end after 10-15 thousand km of service. The domestic method for such cases has not been developed, and therefore we must admit here too: oils are compatible in this case. Finally, one more promised result. It is also very indicative, although it is unlikely to surprise anyone. Since the initial alkalinity of flushing oils is always lower than that of motor oils - in our case - 4.01 and 2.97 - the addition of 10% "flushing" to fresh oil reduces its ability to neutralize oxidation products. Let's say for a pair of Shell (90%) and Luxoil (10%) from 8.78 to 8.00. But does this mean that it is harmful to flush the motor, because the alkaline supply is also low in mining?
In a word, tests, as often happens, having answered one main question, posed many others. With a fair degree of certainty, one can only repeat: a small admixture of one oil in another during its replacement cannot "screw up" the engine, no matter what the "experts" may say. But we are even more confident that the compatibility check in various aspects should be continued. So wait for the next publications.

* The absolute values \u200b\u200bof alkalinity obtained here cannot be compared with previously published results for these and other oils due to the methodological features of determining the indicator in this and previous works.

Spectrol Super Universal; API SF / CC; SAE 15W40.

Sintec; API SF / CC; SAE 15W40.

Esso Ultra API; SJ / CF; SAE 10W40.

Valvoline Dura Blend; API SJ / CF; SAE 10W40.

Castrol GTX Magnatec; API SL / CF; SAE 5W40.

Shell Helix Ultra; API SL / CF; SAE 5W40.